The properties of titanium have long been recognized as a light, strong, and corrosion resistant metal, which has lead to many different approaches over the past few decades to extract titanium from its ore. These methods were summarized by Henrie [1]. Despite the many methods investigated to produce titanium, the only methods currently utilized commercially are the Kroll and Hunter processes [2, 3]. These processes utilize titanium tetrachloride (TiCl4) which is produced from the carbo-chlorination of a refined titanium dioxide (TiO2) according to the reaction:TiO2(s)+2Cl2(g)+2C(s)→TiCl4(g)+2CO(g).In the Kroll process [2] TiCl4 is reduced with molten magnesium at ≈800° C. in an atmosphere of argon. This produces metallic titanium as a spongy mass according to the reaction:2Mg(l)+TiCl4(g)→Ti(s)+2MgCl2(l)from which the excess Mg and MgCl2 is removed by volatilization, under vacuum at ≈1000° C. The MgCl2 is then separated and recycled electrolytically to produce Mg as the reductant to further reduce the TiCl4. In the Hunter process [3,4] sodium is used as a reductant according to the reaction:4Na(l)+TiCl4(g)→Ti(s)+4NaCl(l)The titanium produced by either the Kroll or Hunter processes must not only be separated from the reductant halide by vacuum distillation and/or leaching in acidified solution to free the titanium sponge for further processing to useful titanium forms, but also require the recycling of the reductant by electrolysis. Because of these multiple steps the resultant titanium is quite expensive which limits its use to cost insensitive applications.
The US Bureau of Mines performed extensive additional investigations [1,5-8] to improve the Kroll and Hunter processes. Many other processes have been investigated that include plasma techniques [9-13], molten chloride salt electrolytic processes [14], molten fluoride methods [15], the Goldschmidt approach [16], and alkali metal-calcium techniques [17]. Other processes investigated without measurable success have included aluminum, magnesium, carbothermic and carbo-nitrothermic reduction of TiO2 and plasma reduction of TiCl4[18]. Direct reduction of TiO2 or TiCl4 using mechanochemical processing of ball milling with appropriate reductants of Mg or calcium hydride (CaH2) also have been investigated [19] without measurable success. Kroll, who is considered as the father of the titanium industry [20] predicted that titanium will be made competitively by fusion electrolysis but to date, this has not been realized.
An electrolytic process has been reported [21] that utilizes TiO2 as a cathode and carbon or graphite as the anode in a calcium chloride electrolyte operated at 900° C. By this process, calcium is deposited on the TiO2 cathode, which reduces the TiO2 to titanium and calcium oxide. However, this process is limited by diffusion of calcium into the TiO2 cathode and the build-up of calcium oxide in the cell, which limits operating time to remove the calcium oxide or replacement of the electrolyte. Also the TiO2 cathode is not fully reduced which leaves contamination of TiO2 or reduced oxides such as TiO, mixed oxides such as calcium titanante as well as titanium carbide being formed on the surface of the cathode thus also contaminating the titanium. Thus, current TiO2 cathode electrolytic processes are no more commercially viable than earlier electrolytic processes.